VM66154: GUEST WITH ABSOLUTE LIMITHARD LIMITED INACCURATELY

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• When a guest is performing heavy I/O it may not be limited to
  the guest's ABSOLUTE LIMITHARD SETTING.
  Not limiting a guest accurately can negatively impact other
  guest's consumption capability, particularly in a CPUPOOL.
  

Local fix

• local patch is available upon request
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of SET SHARE LIMITHARD or CPU POOL *
  *                 limiting of guest CPU utilization            *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  ****************************************************************
  * RECOMMENDATION: APPLY PTF                                    *
  ****************************************************************
  SHARE with the LIMITHARD option and RESPOOL (CPUPOOL) limiting
  may not work correctly for all work loads.
  
  A guest with a hard limit (SET SHARE ... LIMITHARD) or that is a
  member of a CPU pool with a CPU limit (LIMITHARD or CAPACITY)
  might not be limited appropriately. While this could occur for
  several reasons, the most likely is that the guest has a high
  degree of I/O activity. While I/O is in progress, a guest holds
  a share of a lock, which prevents it from being added to the
  Limit List. As long as at least one guest I/O operation is in
  flight this condition will prevail. Eventually, when there is no
  active I/O, the guest will be limited, potentially for a long
  period of time, during which it will be completely unresponsive.
  
  In scenarios, such as the one described above, CP work
  dispatched on a guest VMDBK requires a critical lock, or more
  than one critical lock.  Each time one of the locks is obtained,
  a count of critical processes (VMDCTCRT) is incremented.  It is
  decremented when the lock is released.  The VMDBK is not limited
  when that critical count is non-zero so that release of the lock
  isn't delayed for a long time while the guest is limited.
  
  When a guest is doing things that require much CP involvement
  and that CP work frequently requires critical locks, the
  critical count can stay non-zero for an extended amount of time.
  If that happens and the guest is limited by the SHARE LIMITHARD
  option or by RESPOOL limiting, the VMDBK may be withheld from
  the limit list even though it is due to be limited.  Eventually,
  when the count goes to zero, the VMDBK will be limited for an
  extended amount of time to make up for the excess CPU it used
  when it should have been limited.  This can cause erratic
  response times from the guest and can lead to guest time-outs.
  

Problem conclusion

• The dispatcher and scheduler are changed to dispatch CP work on
  behalf of a guest VMDBK that is being otherwise limited.  The CP
  work will be allowed to execute but the VMDBK will not be run
  under SIE.
  
  To accomplish this the actual addition of a VMDBK to the limit
  list is moved from the scheduler to the dispatcher.  The
  dispatcher will only add the VMDBK to the limit list when no
  more CP work is stacked on it.  Also, the VMDBK is temporarily
  taken off the limit list if it is being made ready because CP
  work has been stacked on it.
  

Temporary fix

• FOR RELEASE VM/ESA CP/ESA R640 :
  PREREQ: VM65615
  CO-REQ: NONE
  IF-REQ: NONE
  FOR RELEASE VM/ESA CP/ESA R710 :
  PREREQ: VM65715
  CO-REQ: NONE
  IF-REQ: NONE
  

Comments

• ×**** PE20/06/10 FIX IN ERROR. SEE APAR VM66400  FOR DESCRIPTION
  

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

  UM35444 UM35445

Modules/Macros

• HCPCPA   HCPCPU   HCPDSP   HCPDSW   HCPLMC   HCPSCH   HCPSCI
  HCPSCK   HCPSRN   HCPSTK   HCPSTL   HCPSTM   HCPVMDBK
  

Fix information

• Fixed component name

  VM CP

• Fixed component ID

  568411202

Applicable component levels

• R640 PSY UM35444

     UP19/05/16 P 2001

• R710 PSY UM35445

     UP19/05/16 P 1902

Fix is available

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